This proposal will determine whether differences in prmary amino acid sequence in the heavy chains of immunoglobulins account for the dual localization of immunoglobulins on plasma membranes and in serum. Radioactive serum and membrane IgM will be obtained by biosynthetic labeling with chosen radioactive amino acid precursors using appropriate lymphoid cell populations from mouse spleen. Methods for demonstrating the presence of the known C-terminal amino acid sequence of the heavy chain will be tested with the serum IgM. They will then be applied to the membrane IgM. In particular, the presence of a carboxypeptidase labile tyrosine, the presence of a cyanogen bromide octapeptide labeled with cysteine and tyrosine will be assessed. The presence of a sequence unique to membrane IgM heavy chain will be assessed by fragmentation by cyanogen bromide and gel electrophoresis. Other specific cleavages and separation techniques will be applied as necessary. Preliminary experiments indicate that the delta chain of membrane IgD as previously isolated in this laboratory and others may be slightly smaller than the delta chain on the membrane. Experiments will be performed to attempt to prevent the apparent degradation of the native delta chain. When conditions for producing intact IgD and degraded IgD have been established they will be used to compare the membrane activity of the IgD by an aggregation assay and by a liposome insertion assay. It is anticipated that the degraded IgD has lost the C-terminal fragment which makes it membrane active.